This invention relates to an information data transfer system which can be applied to a reproducing device for an optical disk such as a CD (compact disk), and more particularly to a system for controlling the error correction ability based on occurrence of a track jump.
A CD which was originally used as an audio recording medium becomes more widely used by utilizing the feature of high-speed access and the large recording capacity of the optical disk and it is now applied not only in the field of music but also in the computer field and video field.
In a disk reproducing device, an EFM signal recorded on the disk is optically read by a pickup, the signal is amplified by an RF circuit, demodulated, de-interleaved and subjected to the correction process by a signal processing circuit, and then output as audio data or computer data. The above operations are controlled by a system controller. The signal processing circuit includes a PLL circuit, EFM demodulating circuit, subcode demodulation circuit, memory control circuit, memory, correction circuit and data output circuit.
A bit clock is extracted from the EFM signal in the PLL circuit, and the EFM demodulation and subcode demodulation are effected by use of the bit clock. In EFM data, one block which is called one frame is constructed by 588 bit clocks, and one symbol of subcode and 32 symbols of data are included in one frame. Among data which has been subjected to the EFM demodulation, one symbol of subcode is supplied to the subcode demodulating circuit and the 32 symbols of data are written into the memory by the memory control circuit and then corrected in the correction circuit.
In the CD system, an error correction code called CIRC (Cross Interleaved Reed-Solomon Code) is used. The feature of CIRC is that Reed-Solomon codes (C1, C2) of two stages which are combined in an interleaved form are used to enhance the correction ability.
In the series of C1 and C2, parities necessary for correction, that is, P-parity and Q-parity of four symbols are attached in the respective frames. That is, symbols of one-fourth of the 32 data symbols in total are used for correction.
First, based on the four symbols of the P-parity in the C1 series, correction of random errors occurring by jitter or interference between codes is made in the remaining 28 data symbols. In the next C2 series, a long-range interleaving process is effected for a maximum of 108 frames to cope with a long and large burst error which occurs when the synchronized operation is not attained because of malfunctioning of the disk reproducing device, the servo operation is disturbed, or a relatively large defect occurs on the disk.
The interleaving process can be easily realized by use of the memory.
The correction process is explained more in detail with reference to FIG. 1.
The number of symbols which can be corrected by use of the four symbols of the P-parity in the C1 series is up to two symbols. When correction of two symbols in the C1 series is made or when an error of three of more symbols occurs and cannot be corrected, a pointer is set in the frame. Then, correction is made again in the C2 series on the assumption that all of the symbols of the frame in the C1 series in which the pointer is set contain error factors. The number of symbols contained in the Q-parity used in the C2 series is four, but since the position of the error is almost determined by the pointer information set at the time of correction for the C1 series ,a maximum of four symbols can be corrected.
As described above, the signal is restored with high fidelity by effecting the correction process twice for the C1 series and C2 series in the error correction circuit of the signal processing circuit. However, as shown in FIG. 2, since the C2 correction process is effected based only on the pointer information attached in the C1 series in the conventional system if a track jump occurs in the burst error of 16 frames occurring in the C1 series, data which is not originally required to be corrected is also subjected to the correction process. The track jump means that the pickup is moved to a different track by the influence of a scratch made on the disk while a certain track is being reproduced.
Therefore, there occurs a problem that the quadruplex correction ability used for effectively coping with occurrence of the long and huge burst error may adversely cause erroneous correction when the track jump has occurred.